Plastic materials such as polymeric films have been widely used for packaging various food and non-food products. In order to ensure proper preservation of products packaged in such polymeric films, it is necessary to provide the films with barriers against transmission of air, moisture, deleterious flavors, etc. Unmodified polymeric films, however, typically lack sufficient gas and moisture barrier characteristics needed for proper packaging requirements. For example, polyolefin films are particularly preferred in the manufacture of packaging films due to their low cost and ease of manufacture. Such films, however, typically permit the transmission of oxygen and water vapor from the outside of the film to the inside of the package made up of the film. As will be recognized by those skilled in the art, transmission of oxygen and water vapor through food packaging materials promotes deterioration of the foods packaged therein.
The use of a metallized film to improve the barrier properties of flexible packaging films is known in the art. Typically such barrier films are formed by depositing a thin layer of a metal, most typically aluminum, onto a thermoplastic substrate oriented prior to metal deposition. A requirement of these polymer substrates or films is that they be uniaxially oriented, i.e., stretched in one direction, or, most often, biaxially oriented, i.e., stretched in both a longitudinal direction and the transverse direction, before being metallized. For example, U.S. Pat. No. 5,283,118 to Murakami et al., the disclosure of which is incorporated herein by reference, discloses metallized oriented films having water vapor transmission values (WTR) between 0.011 to 0.10 g/100 in2/24 hours and oxygen transmission values (O2TR) between 1 to 30 cm3/100 in2/24 hours. The metallization is carried out by vacuum-vapor deposition of aluminum and aluminum alloys on the surface of a biaxially oriented propylene homopolymer or copolymer substrate. U.S. Pat. No. 5,698,317 to Kurokawa et al., the disclosure of which is incorporated herein by reference, teaches that oxygen transmission rates between 0.4 to 1.14 cm3/100 in2/24 hours for films can be obtained by vacuum vapor-deposition of a metal onto the surface of biaxially oriented multilayer film packaging webs having a polypropylene-based composition. U.S. Pat. No. 5,827,615 to Touhsaent et al., the disclosure of which is incorporated herein by reference, discloses an ethylene/vinyl alcohol (EVOH) surface biaxially oriented film substrate having an aluminum coating which is applied by conventional vacuum deposition. The resulting films have excellent barrier properties, i.e., WVTR values of less than 0.1 g/100 in2/24 hours and O2TR values of less than 0.1 cm3/100 in2/24 hours.
A need exists for a metallized packaging material having sufficient gas and moisture barrier properties formed from a non-oriented substrate.